*{
    box-sizing: border-box;
}
html,body{
    height: 100%;
}
body{
    margin: 0;
    background-image: radial-gradient(ellipse, #333, #666, #bbb);
}
#nav{
    height: 10vh;
    background-color: cornflowerblue;
    padding-top: 2vh;
    z-index: 1000;
}
#nav div{
    text-align: center;
    height: 5vh;
    font-size: 4vh;
    color: white;
    margin: 0 auto;
    letter-spacing: 0.5vw;
}
#tiaojian{
    font-size: 3vh;
    color: white;
    text-align: center;
}
table{
    border-collapse: collapse;
}

@media screen and (orientation:landscape){
    table{
    width: 60vw;
    height: 60vw;
    margin: 15vh auto;
}
}
@media screen and (orientation:portrait){
    table{
    width: 60vh;
    height: 60vh;
    margin: 15vh auto;
}
}


td{
    background-repeat: no-repeat;
    background-position: center;
    background-size: cover;
}

#butong{
    display: block;
    width: 100%;
    height: 100%;
    background-size: cover;
    background-repeat: no-repeat;

}